Revisiting the Humidity Ramp Protocol for Assessing Human Heat Tolerance Limits

BackgroundHumidity ramp protocols are widely used to assess human heat tolerance limits, but the impact of ramp temporal structure (e.g., step duration) on estimated critical environmental limits (CELs) remains unclear. This study integrated theoretical modeling and empirical testing to assess these effects on apparent core temperature (Tcr) inflection points. MethodsA first-order thermal model described Tcr dynamics during stepwise humidity changes at fixed dry-bulb temperature (Tdb), with analytical solutions for increments of duration {Delta}t and sensitivity analyses across relevant time constants ({tau}). Twenty-six healthy young adults (14 males, 12 females) completed randomized trials at Tdb=42 {degrees}C: (1) slow-ramp (4-hour equilibration at 40% RH, then +6% RH/hour for 2 hours followed by +3% RH/hour; RH range: 40-61%) and (2) aggressive-ramp (30 min equilibration, then +2% RH every 5 min; RH range: 28-88%). Rectal and skin temperatures, heart rate, and perceptual ratings were monitored continuously. ResultsWhen {Delta}t/{tau} <<1, thermal disequilibrium accelerated Tcr rises, yielding prematurely low CELs; dwell times [&ge;] 60 min/step permitted near-equilibrium and higher thresholds. Aggressive-ramp CELs were significantly lower than slow-ramp (males: 29.9{+/-}1.6 {degrees}C vs. 33.4{+/-}0.5 {degrees}C; females: 30.3{+/-}0.9 {degrees}C vs. 33.8{+/-}0.5 {degrees}C), with downward shifts of 3.4{+/-}1.9 {degrees}C and 3.5{+/-}0.9 {degrees}C, respectively. ConclusionRapid humidity increments systematically underestimate heat tolerance due to thermal lag. Accurate CEL determination requires prolonged stable exposures (gold standard) or slow ramps ensuring sufficient equilibration ({Delta}t [&ge;] 60 min/step). Our findings reveal a core limitation of aggressive-ramp protocols and offer a framework for improved assessment of human environmental compensability. NEW & NOTEWORTHYThis study reveals how ramp temporal structure affects heat tolerance assessment. Rapid humidity increments in aggressive-ramp protocols cause premature underestimation of critical environmental limits (CELs) due to thermal disequilibrium. In contrast, prolonged dwell times ([&ge;] 60 min/step) in slow ramps allow near-equilibrium conditions, resulting in higher and more accurate CELs. These findings emphasize the importance of equilibration time in defining heat tolerance and provide a more reliable approach for assessing heat stress in extreme environments.